Hyperfine interaction and magnetoresistance in organic semiconductors

TL;DR

This paper investigates whether hyperfine interactions cause organic magnetoresistance (OMAR), using experiments and models, and finds that excitonic effects are unlikely to be the main cause of OMAR.

Contribution

It critically evaluates the excitonic pair mechanism model for OMAR and provides experimental evidence against excitonic effects being the primary cause.

Findings

Hyperfine interaction alone cannot fully explain OMAR.

OMAR is weakly dependent on exciton formation ratio.

Excitonic effects are likely not the main origin of OMAR.

Abstract

We explore the possibility that hyperfine interaction causes the recently discovered organic magnetoresistance (OMAR) effect. Our study employs both experiment and theoretical modelling. An excitonic pair mechanism model based on hyperfine interaction, previously suggested by others to explain magnetic field effects in organics, is examined. Whereas this model can explain a few key aspects of the experimental data, we, however, uncover several fundamental contradictions as well. By varying the injection efficiency for minority carriers in the devices, we show experimentally that OMAR is only weakly dependent on the ratio between excitons formed and carriers injected, likely excluding any excitonic effect as the origin of OMAR.